Exercises: Melting point, boiling point and solubility

Define the terms melting point and boiling point. State the melting and boiling points of pure water at normal atmospheric pressure in °C.

The table gives data for four substances.

State the physical state (solid, liquid or gas) of each substance at $25\ °\text{C}$.

Define the terms solute, solvent and solution. Identify each in a mixture of salt dissolved in water.

Using the data below, answer the questions.

1. a) Which substance is a liquid at $0\ °\text{C}$?

1. b) Which substance is a gas at $−100\ °\text{C}$?

1. c) State the physical state of sulfur at $200\ °\text{C}$.

1. d) Calculate the temperature range over which mercury is a liquid.

The solubility of potassium nitrate in water at different temperatures is shown below.

1. a) State how the solubility changes as temperature increases.

1. b) Estimate the mass of potassium nitrate that dissolves in $200\ \text{g}$ of water at $60\ °\text{C}$.

1. c) A saturated solution containing $100\ \text{g}$ of water at $80\ °\text{C}$ is cooled to $20\ °\text{C}$. Calculate the mass of crystals that form.

A student measures the melting point of a sample of a solid and finds that it melts gradually between $78\ °\text{C}$ and $85\ °\text{C}$. The pure substance has a sharp melting point of $88\ °\text{C}$.

1. a) Is the sample pure? Explain.

1. b) State the effect of impurities on the melting point.

1. c) Explain why grit and salt are spread on roads in Ireland in winter.

A heating curve for a pure substance is recorded by heating a solid at a steady rate. The temperature rises from $20\ °\text{C}$ to $80\ °\text{C}$, stays at $80\ °\text{C}$ for $5$ minutes, rises to $218\ °\text{C}$, stays at $218\ °\text{C}$ for $12$ minutes and then rises again.

1. a) Identify the melting point and the boiling point.

1. b) Explain why the temperature stays constant during the flat sections even though heat is still supplied.

1. c) Explain why the flat section at the boiling point is longer than that at the melting point.

1. d) Name the state or states present during each flat section.

Two white solids, P and Q, look identical. P has a melting point of $801\ °\text{C}$, is soluble in water, and its solution conducts electricity. Q has a melting point of $55\ °\text{C}$, is insoluble in water, and does not conduct electricity in any state.

1. a) Suggest the type of substance (ionic or molecular) for each, justifying your answer.

1. b) Describe how you would separate a mixture of P and Q, naming the processes.

1. c) Explain why P has a much higher melting point than Q in terms of the forces between particles.

The solubility of a gas such as oxygen in water decreases as temperature increases, whereas most solids become more soluble.

1. a) Use this fact to explain why fish in rivers can be at risk during a very hot summer.

1. b) Explain why bubbles form in a glass of tap water left to warm on a counter.

1. c) A student claims that "all solids dissolve better in hot water". Use the data for potassium nitrate (solubility $32\ \text{g}/100\ \text{g}$ at $20\ °\text{C}$ and $169\ \text{g}/100\ \text{g}$ at $80\ °\text{C}$) to calculate how many times more soluble it is at $80\ °\text{C}$, and suggest why the student's claim is too general.